the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Interannual Variations of Terrestrial Water Storage in the East African Rift Region
Abstract. The US-German GRACE (Gravity Recovery and Climate Experiment, 2002–2017) and GRACE-FO (GRACE-Follow-On, since 2018) satellite missions observe terrestrial water storage (TWS) variations. Over twenty years of data allow for investigating interannual variations beyond linear trends and seasonal signals. However, the origin of observed TWS changes, whether naturally caused or anthropogenic, cannot be determined solely with GRACE and GRACE-FO observations. This study focuses on the East African Rift region region around lakes Turkana, Victoria, and Tanganyika. It aims to characterise and analyse the interannual TWS variations together with surface water and meteorological observations and determine whether natural variability or human interventions caused these changes.
To this end, we apply the STL method (Seasonal Trend decomposition based on Loess) to separate the TWS signals into a seasonal signal, an interannual trend signal, and residuals. By clustering these interannual TWS dynamics for the African continent, we define the exact outline of the study's region.
In this area, a TWS decrease until 2006 was followed by a steady increase until around 2016, and Africa's most significant TWS increase occurred in 2019 and 2020. We found that besides precipitation and evaporation variability, surface water storage variations in the large lakes of the region explain large parts of the TWS variability. Storage dynamics of Lake Victoria regulated by the Nalubaale Dam alone contribute up to 50 % of the TWS changes. Satellite altimetry reveals the anthropogenically altered discharge downstream of the dam. It thus indicates that human intervention in the form of dam management at Lake Victoria substantially contributes to the TWS variability seen in the East African Rift region.
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Status: open (until 17 May 2024)
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RC1: 'Comment on egusphere-2024-641', Vagner Ferreira, 09 Apr 2024
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The study “Interannual Variations of Terrestrial Water Storage in the East African Rift Region” addresses an interesting topic and provides some valuable insights. However, several issues need to be addressed before the manuscript can be considered for publication. I recommend major revisions based on the following main points and some other minor comments presented below.
Main points:
1. The authors state that “human intervention in the form of dam management at Lake Victoria substantially contributes to the TWS variability” (lines 15-16); however, they didn’t provide a clear estimation of the magnitude of this contribution. It would be interesting to see the relative contribution of natural variability and human interventions to the observed TWS fluctuations.
2. The study proposes a clustering approach to identify the East African Rift region as having similar interannual TWS dynamics. However, the justification for focusing on this specific region could be further improved by providing a stronger rationale for selecting this study area. The manuscript could highlight the East African Rift region's unique hydrological characteristics, ecological significance, or socio-economic importance.
3. Although the study compares TWS variations with precipitation, evapotranspiration, and surface water storage in the major lakes, the analysis of the underlying drivers remains somewhat superficial. The study could provide more information about the potential mechanisms that link these factors to TWS variability in the region (e.g., land use/land cover changes, soil moisture dynamics, groundwater recharge, and human water abstractions). A more comprehensive discussion of these drivers would beef up the interpretations and conclusions of the study.
4. The study cites some relevant literature; however, it could improve by discussing how the proposed study’s findings compare to or advance previous research on TWS variability in the East African Rift region. The paper would benefit from a more thorough synthesis of the existing knowledge and a clearer articulation of this study’s novel contributions.
5. The study lacks a thorough assessment of the uncertainties associated with the GRACE/GRACE-FO data, the precipitation and evapotranspiration datasets, and the surface water storage estimates. It would be interesting to see a more detailed description of the potential sources of error and their implications for the results. Also, the authors could elaborate more on the limitations, such as the coarse spatial resolution of GRACE data and the lack of ground-based validation data. These limitations could be explicitly acknowledged and discussed.
6. The current conclusion section is somewhat vague and does not fully address the broader implications of the findings for water resources management, ecosystem conservation, or climate change adaptation in the region (conditioned to the rationale for selecting the study area as per comment 2). The authors could elaborate on the potential applications of the study’s findings.
Minor comments:
7. Between lines 35-40, where it is “Niger Basin in West Africa,” it should be “Volta Basin in West Africa” in the context of the sentence.
8. Lines 134-137: The description of the water occurrence map processing is unclear. Please provide more details on how the 95% occurrence threshold was determined and how it affects the estimation of lake surface areas.
9. Lines 139-143: Please discuss the limitations of the surface water storage analysis based on a simplified relationship between lake level and area changes based on empirical cumulative distribution functions (ECDF). What could be the potential uncertainties it introduces in the storage estimates? For example, the monotonic and continuous relationship between lake level and area might not always be the case in reality. Lakes with complex bathymetry or irregular shorelines may exhibit non-monotonic or discontinuous relationships between level and area. However, the ECDF approach can handle outliers or anomalies in the input data more robustly than a linear regression used by Ferreira et al. (2018).
10. Lines 240-247: The discussion of the differences between the two SPEI datasets seems speculative. Please provide more evidence to support the claim that the divergence after 2008 is caused by differences in precipitation data rather than PET estimation methods.
11. Lines 290-295: The description of the Nalubaale Dam and its impact on Lake Victoria's water levels is incomplete. Please provide more information on the characteristics of the dam (e.g., operating rules) and downstream effects on the Victoria Nile and other water bodies. A study area section presenting the East African Rift Region would be useful.
12. Lines 314-315: Please provide a more rigorous assessment of the data quality and its impact on the correlation analysis.
13. Lines 367-368: That concluding statement seems too broad and not fully supported by the analysis. Please refine this conclusion and provide a more nuanced interpretation of the relative contributions of natural and anthropogenic factors to TWS variability.
14. Please revise the English since there are several issues (e.g., Line 5 shows “region region”, Line 92 shows “We analyses…”)Citation: https://doi.org/10.5194/egusphere-2024-641-RC1
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